Hint (SQL)
In various SQL implementations, a hint is an addition to the SQL standard that instructs the database engine on how to execute the query. For example, a hint may tell the engine to use or not to use an index (even if the query optimizer would decide otherwise).
Implementation[]
Different database engines use different approaches in implementing hints.
- MySQL uses its own extension to the SQL standard, where a table name may be followed by USE INDEX, FORCE INDEX or IGNORE INDEX keywords.[1]
- Oracle implements hints by using specially-crafted comments in the query that begin with a + symbol, thus not affecting SQL compatibility.[2]
- Postgres Plus Advanced Server (a proprietary version of PostgreSQL from EnterpriseDB) offers hints compatible with those of Oracle.[3][4]
- Microsoft SQL Server offers hints via the OPTION keyword [5]
See also[]
- Query optimizer
- Query plan
References[]
- ^ MySQL 5.5 Reference Manual: 12.2.9.3 Index Hint Syntax
- ^ Mike Ault: Oracle SQL Hints Tuning
- ^ Postgres Plus Advanced Server Performance and Scalability Guide: Query Optimization Hints
- ^ "Postgres Plus Advanced Server Oracle Compatibility Developer's Guide: Optimizer Hints". Archived from the original on 2014-04-19. Retrieved 2013-06-24.
- ^ MSSQL 2017 Documentation
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